------------------------------------------------------------------ Psychology. Among humans, is the purpose of sex to have children or to provide pleasure? Two psychologists contend that evolution suggests the latter, and the implications, not surprisingly, can be controversial. ------------------------------------------------------------------

When a group of foreign journalists passed through Seattle on a tour of the United States recently, they remarked that America was pretty much as expected.

Back home in Europe they already had watched our movies and television series, listened to our music, worn our fashions, followed our fads and been affected by our politics. They knew Arnold and Sly, Hillary and Newt.

From gays in the military to the Christian right, from Clarence Thomas to Bob Packwood, from anti-porn feminist Andrea Dworkin to Hollywood madam Heidi Fleiss, from Internet porn to the furor over federal grants to the arts, they saw us as . . . odd.

For scientists, both European casualness and American moralism about sex raises an interesting question. Why are humans so continually preoccupied with the subject, especially when many mammals mate only periodically or seasonally, and often hurriedly? What is the purpose of sex?

To have children?

It's more than that, argue two University of California at Los Angeles psychologists in a new book called "With Pleasure: Thoughts on the Nature of Human Sexuality" (Oxford University Press, $25).

Humans evolved to have sex not only for procreation but for pleasure, argue Paul Abramson and Steven Pinkerton, the latter a native of Seattle. As social animals, we use it to enhance love, to make peace after fights, to relieve tension, for social advantage, for recreation and for income.

"Pleasure has evolutionary advantages," said Abramson.

Natural and necessary

This distinction is important, they contend. If sex is seen only to produce offspring, then anything that does not contribute to that goal - oral and anal sex, masturbation, homosexuality, pornography and prostitution - can be seen as morally offensive, unnecessary and subject to censure, legal prohibition or religious injunction.

If, on the other hand, the pursuit of pleasure is seen as natural and necessary, then laws should shift from prohibiting nonreproductive sex to merely regulating it.

Regulation, the duo said, should focus on health: requiring prostitutes, for example, to use condoms to prevent sexually transmitted diseases.

They argue sexual activity between consenting adults should be permitted without guilt, including gay sex, prostitution and strip clubs.

But sex with minors below the age of consent, with unwilling partners or which intrudes without consent (television or pornography on the Internet that cannot be electronically locked out, for example) should be restricted.

Admitting that sexual pleasure is a legitimate, evolutionary part of human life also implicitly permits birth control as moral, the pair argue. "If sex is for pleasure, a win-win solution is to build better condoms," said Abramson.

The evolution of pleasure

Humans are not the only animals that use sex for pleasure, the psychologists note. Male dolphins display homosexual and masturbatory behavior. Males and females stroke each others' genitals with beak and flipper.

Primates are highly sexual. The bonobo, or pygmy chimp, indulges in homosexual sex by both males and females, oral sex, masturbation, and sex between adults and juveniles.

The psychologists theorize that humans, as social animals who choose mates for a variety of reasons, evolved sexual pleasure. Natural selection may have favored ancestors who developed pleasure centers - such as the female clitoris - because they might have mated more or been more desirable, and thus left more offspring.

Another theory is that males and females, with the same basic body plan, start similar structures while a fetus but don't develop them. Thus, males have nipples without breasts, while a clitoris is the female analog to the penis.

Civilization's boundaries

So does the evolution of sexual pleasure justify indulgence in it? Or is this just wishful 1960s sexual liberation dressed up in 1990s scientific rationalism? Isn't one of the points of civilization to curb our animal instincts?

Not if the instincts have an evolutionary purpose still valid today, the pair contended in an interview. While the unrestrained pursuit of sexual pleasure can hurt and disrupt, consensual pleasures from touching to intercourse is not just fun, it has a social function - from holding marriages together to soothing grief, they said.

Prohibition that runs against our genetic programming as a "well-adapted ape" just creates problems, said Pinkerton. "What you do if you ban activities is push them underground. Sex for pleasure is never going to disappear."

For the record, both men are heterosexuals who are active in AIDS sociological research, having recently made a mathematical case that pursuit of an AIDS vaccine will probably save fewer lives than encouragement of condom use. Abramson has also worked as an expert witnesses for pornography production companies in court cases.

Abramson has three children and is in the process of divorce; Pinkerton is married with no children.

The authors' theories

Some of the provocative arguments the pair makes:

-- Visual pornography has never found much of a market among women, who tend to prefer the relationship fantasies of romantic novels and movies or erotic literature. But porn has historically been popular as a masturbatory aid and source of sexual education for men, 94 percent of whom have masturbated, according to one poll cited in the book. In the same poll, 63 percent of women said they had masturbated.

-- This male desire for pornography tends to drive new communication technologies. The printing press was seized upon immediately to reproduce erotic tales. Photography got a boost from soldier demands for nude pictures in the American Civil War. The ability to see X-rated videos without having to go to an adult theater fueled early purchases of VCRs. Cyberporn encouraged early consumer exploration of the Internet. Sex, the authors predict, will help push the development of virtual-reality technology - even though women will remain relatively uninterested.

-- Changing American attitudes toward sex from prohibition to regulation would help combat sexually transmitted diseases. Pornography or phone sex and masturbation "is sex without the threat of a sexually transmitted disease, including AIDS," they write. "From an epidemiologic standpoint, it therefore has substantial social value." Similarly, they point out, prostitutes in Nevada's regulated brothels have not experienced AIDS because of the requirement to use condoms. And recent gay pornography may serve an educational function by showing partners using condoms.

-- The sex industry in main is not coercive of or exploitative of women. The pair argue some of its most prominent executives are women, that it is a field in which women can earn good money, and that it should not be perceived as degrading, but rather as fulfilling natural human needs. The psychologists do acknowledge the need for regulation of child, rape or violence pornography, which they say is a tiny fraction of the industry.

"Don't base laws on what offends the most sensitive portion of our society," added Pinkerton. "The idea that if it offends anybody it is bad went out 50 to 100 years ago."

But what about love?

"With Pleasure" joins a growing number of books looking at the possible evolutionary roots of human social behavior, including monogamy and courtship, the large size of human female breasts and male genitals (compared to those of primates) and even the appeal of high heels.

Astronomer Carl Sagan has theorized that high heels not only accentuate the female breasts and buttocks, but they also quell male anxiety by making it hard for the woman to walk, sending a psychological signal that she does not plan to flee.

Abramson and Pinkerton expect their interpretation to generate controversy, which they said is almost unavoidable for a sex researcher.

And it leaves mostly unaddressed the question of love. Did that evolve? What is its relationship to pleasure? And should a loving relationship be regulated differently than a commercial one?